The present invention relates to a method for producing a strip suitable for further processing to yield a grain oriented electrical steel with low core loss and high magnetic permeability whereby the steel is produced from a steel melt which is first cast as a thin sheet or strip. It is subsequently processed to produce a finished strip of the desired thickness. The finished strip is preferably further subjected to at least one annealing treatment wherein the magnetic properties are developed, making the steel sheet of the present invention suitable for use in electrical machinery such as motors or transformers.
In particular, the present invention relates to a method of producing a strip suitable for further processing to yield a cube-on-edge oriented electrical steel strip and sheet. Cube-on-edge orientation is designated (110)[001] in accordance with the Miller Indices. In particular, the present invention provides a method of producing a (110)[001] grain oriented electrical steel from a thin strip such as a continuously cast thin strip. This thin cast strip is processed to promote recrystallization from the surface layer of the strip (S=0) into the quarter thickness of the strip (S=0.2 to 0.3). As used herein, the term S is used as a reference to a planar position through strip or sheet thickness. In the form used in this disclosure, the position of S=0 refers to the planar thickness position located at the very surface, or 0% of the thickness, of the strip; S=0.2-0.3 refers to the planar position located between 20% and 30% of the thickness of the strip; and, S=0.5 refers to the planar thickness position located halfway through the thickness of the strip.
Grain oriented electrical steels are widely used as the magnetic core material in a variety of electrical machinery and devices, particularly in transformers where the highly directional magnetic properties developed in the sheet direction parallel to the rolling of the sheet can be utilized. Typical applications for grain oriented electrical steels include magnetic cores in power transformers, distribution transformers, large generators and a wide variety of small transformers. Core configurations can include sheared flat laminations, wound cores, segmental laminations for large generators, and some xe2x80x9cExe2x80x9d and xe2x80x9cIxe2x80x9d types.
The performance of grain oriented electrical steels is typically characterized by a magnetic property called core loss, which is a measure of the power loss during magnetization in an alternating current (AC) field. Core loss is the electrical energy that is expended in the core steel without contributing to the work of the device. Core loss is reported in watts per kilogram using the SI system and in watts per pound using the English system. The core loss of a grain oriented electrical steel can be affected by volume resistivity of the sheet and the technical characteristics of the finished sheet such as the sheet thickness, the quality of the (110)[001] crystallographic texture of the sheet and intrinsic and extrinsic factors which affect the domain wall spacing, such as the size of (110)[001] grains in the finished sheet, the presence of a tension imparting coating onto the finished sheet or the application of a secondary treatment such as laser scribing to the surface of the finished sheet.
The production of grain oriented electrical steels requires vigorous and predictable conditions within which to effect secondary grain growth. Two prerequisite conditions for developing a high quality (110)[001] grain orientation are (1) the steel sheet must have a structure of recrystallized grains with the desired orientations prior to the high temperature portion on the final annealing step wherein a process known as secondary grain growth occurs; and (2) the presence of a grain growth inhibitor to restrain primary grain growth in the final annealing step until secondary grain growth is substantially completed. The first precondition requires that the steel sheet and in particular, the surface and near-surface areas of the steel sheet, have a recrystallized grain structure and crystallographic texture appropriate for secondary grain growth. The (110)[001] grains that experience vigorous secondary grain growth are typically located in these surface and near-surface areas of the sheet. The second precondition requires a phase to inhibit primary grain growth while allowing these primary grains to be consumed by growing (110)[001] grains. A dispersion of fine particles, such as manganese sulfides and/or selenides, aluminum nitrides, or both, are effective and well-known means of providing primary grain growth inhibition.
Grain oriented electrical steels are further characterized by the type of the grain growth inhibitors used, the processing steps used and the level of magnetic properties developed. Typically, grain oriented electrical steels are separated into two classifications, conventional (or regular) grain oriented and high permeability grain oriented, based on the level of the magnetic permeability obtained in the finished steel sheet.
The magnetic permeability of grain oriented electrical steels is affected by the quality of the crystal orientation of the finished steel sheet. The processing of oriented electrical steel results in most of the grains being arranged so that edges of the unit cubes comprising each grain are aligned parallel to the rolling direction in a cube-on-edge position with face diagonals aligned in the transverse direction. Because each cube is most easily magnetized along its edge, the [001] direction, the magnetic properties of oriented electrical steels are typically best in the rolling direction. The face diagonal, the [110] direction, of each cube is typically more difficult to magnetize than the cube edge and the cube diagonal, the [111] direction, is generally the most difficult to magnetize. Thus, in a typical grain oriented electrical steel, the magnetic properties are typically best in the rolling direction, poorer at 90xc2x0 to the rolling direction, and poorest at 55xc2x0. The magnetic permeability of grain oriented electrical steels, typically measured at a magnetic field density of 796 A/m, provides a measurement of the quality of the (110)[001] grain orientation in the rolling direction of the finished steel sheet.
Conventional grain oriented electrical steels typically have magnetic permeability measured at 796 A/m of greater than 1700 and below 1880. Regular grain oriented electrical steels typically contain manganese and sulfur (and/or selenium) that combine to form the principal grain growth inhibitor(s) and are processed using one or two cold reduction steps with an annealing step typically used between cold reductions steps. Aluminum is generally less than 0.005% and other elements, such as antimony, copper, boron and nitrogen, may be used to supplement the inhibitor system to provide grain growth inhibition. Conventional grain oriented electrical steels are well known in the art. U.S. Pat. Nos. 5,288,735 and 5,702,539, incorporated herein by reference, describe exemplary processes for the production of conventional grain oriented electrical steel.
High permeability grain oriented electrical steels typically have magnetic permeability measured at 796 A/m of greater than 1880 and below 1980. High permeability grain oriented electrical steels typically contain aluminum and nitrogen that combine to form the principal grain growth inhibitor with one or two cold reduction steps with an annealing step typically used prior to the final cold reductions step. Other additions can be employed to supplement the grain growth inhibition of the aluminum nitride phase. Such additions can include manganese, sulfur and/or selenium, tin, antimony, copper and boron. High permeability grain oriented electrical steels are well known in the art. U.S. Pat. Nos. 3,853,641 and 3,287,183, incorporated herein by reference, describe exemplary methods for the production of high permeability grain oriented electrical steel.
Grain oriented electrical steels are typically produced using ingots or continuously cast slabs as the starting material. Using these conventional production methods, grain oriented electrical steels are processed wherein the starting cast slabs or ingots are heated to an elevated temperature, typically in the range of from about 2192xc2x0 F. (1200xc2x0 C.) to about 2552xc2x0 F. (1400xc2x0 C.), and hot rolled into a strip, typically having a thickness of from about 0.06xe2x80x3 (1.5 mm) to about 0.16xe2x80x3 (4.0 mm), which is suitable for further processing.
Slab reheating dissolves the grain growth inhibitors that are subsequently precipitated to form a fine dispersed grain growth inhibitor phase. The inhibitor precipitation can be accomplished during or after the step of hot rolling, annealing of the hot rolled strip, and/or annealing of the cold rolled strip. Breakdown rolling of the slab or ingot prior to reheating of the slab or ingot in preparation for hot rolling may be employed in the production of grain oriented electrical steels. U.S. Pat. Nos. 3,764,406 and 4,718,951, incorporated herein by reference, describe exemplary prior art methods for the breakdown rolling, slab reheating and hot strip rolling used for the production of grain oriented electrical steels.
In addition, the strip usually undergoes one or more cold reduction steps. The strip is annealed between multiple cold reductions. The end result of this processing is a thin sheet material, typically having a thickness of from about 0.06xe2x80x3 (1.5 mm) to about 0.16xe2x80x3 (4.0 mm), which is suitable for further processing.
Typical, conventional methods used to process grain oriented electrical steels may include hot band annealing, pickling of the hot rolled or hot rolled and annealed strip, one or more cold rolling steps, an annealing step between cold rolling steps and a decarburization annealing step between cold rolling steps or after cold rolling to final thickness. The decarburized strip is subsequently coated with an annealing separator coating and subjected to a high temperature final annealing step wherein the (110)[001] grain orientation is developed.
A strip casting process is advantageous for the production of a grain oriented electrical steel because a number of the conventional processing steps used to produce a strip suitable for further processing can be eliminated. Strip casting apparatuses and methods for the production of carbon steels and stainless steels are well known in the art, e.g., U.S. Pat. Nos. 6,257,315; 6,237,673; 6,164,366; 6,152,210; 6,129,136; 6,032,722; 5,983,981; 5,924,476; 5,871,039; 5,816,311; 5,810,070; 5,720,335; 5,477,911; 5,049,204, all of which are incorporated herein by reference.
When employing a strip casting process, at least one casting roll and, preferably, two counter rotating casting rolls are used to produce a strip that is less than about 0.39xe2x80x3 (10 mm) in thickness and, preferably, less than about 0.20xe2x80x3 (5 mm) in thickness and, more preferably, about 0.12xe2x80x3 (3 mm) in thickness. The processing steps which can be eliminated can include, but are not limited to, slab or ingot casting, slab or ingot reheating, slab or ingot breakdown rolling, hot roughing and/or hot strip rolling. Moreover, with the combined use of hot rolling of a thin cast strip for the production of carbon steel and stainless steels, the amount of hot reduction necessary is minimized.
It is well known in the art, for both carbon steels and stainless steels, that the application of a hot reduction to a thin cast strip can be useful to improve the surface characteristics of the finished strip. A thin cast strip often has shrinkage porosity that must be closed to provide a strip with the desired physical and mechanical properties. In addition, textured casting rolls are commonly used for the direct casting of strip. The surface roughness of the as-cast strip reflects the surface roughness of the casting rolls, making the surface of a cast strip less desirable for many applications where a smooth, high quality surface is required.
The application of strip casting to the production of grain oriented electrical steels differs from stainless steels and carbon steels made using strip casting because of different technical requirements for the grain structure, texture and grain growth inhibition (such as MnS, MnSe, AIN and the like), which are preconditions for producing the desired (110)[001] texture by the process of secondary grain growth. Thus, the present invention provides for the production of a strip suitable for further processing to yield a high quality (110)[001] grain oriented electrical sheet from a thin cast ingot or strip.
The present invention provides a method of producing a strip suitable for further processing to yield a (110)[001] grain oriented electrical steel comprising the steps of:
a. obtaining a cast strip having a thickness of less than or equal to about 0.39 in (10 mm);
b. hot rolling the cast strip;
c. annealing the hot rolled strip; and
d. having a strain/recrystalization parameter, (K*)xe2x88x921, xe2x89xa7about 6500;
wherein,             (              K        *            )              -      1        =            (              T        HBA            )        ⁢          ln      ⁡              [                                            e              .                        0.15                    ⁢                      exp            ⁡                          (                              7616                                  T                  HR                                            )                                ⁢                      ln            ⁡                          (                                                t                  c                                                  t                  f                                            )                                      ]            
THBA is the annealing temperature of the strip (in xc2x0Kelvin),
THR is the hot rolling temperature of the strip (in xc2x0Kelvin),
{dot over (xcex5)} is the strain rate of hot rolling,
ti is the initial thickness of the strip before hot rolling, and
tf is the final thickness of the strip after hot rolling.